Here references to the configurability of a reflector refer to the possibility of intentionally modifying its spatial coverage by adjusting the configuration of the radiation transmitted or received in one or more areas of given direction and width by selectively controlling the reflectivity properties of the reflector. With this type of reflector, it is possible in particular to define configurable multibeam or single-beam antennas.
Clearly, given the multiplicity of mobile telephone systems and high bit rate distribution services, the ability to configure a reflector can impact on the number of antennas on the same site. As a function of the required coverage, the antenna can be configured to obtain radiation in a larger or smaller cell or to illuminate a plurality of cells in different angular sectors. Thus coverages can be modified without changing antennas or their positions. Associated with a configurable reflector, the antenna can be a broadband or multiband antenna.
In areas where there is little interference, in particular in a rural environment, reflectors and associated antennas process only the vertical component of the electromagnetic radiation, the horizontal component being of no particular interest.
However, in urban areas where electromagnetic radiation is liable to suffer numerous kinds of interference, such as unwanted reflections, it is advantageous to be able to process vertical polarization and horizontal polarization simultaneously so as to be able to recover whichever of the two signals has the higher power.
With frequency selective surfaces (FSS), tackling reflection problems by processing two orthogonal polarizations has been addressed by the production of cruciform dipole arrays producing the same reflection coefficient in both polarization directions (see V. A. Agrawal, W. A. Imbriale, “Design of a Dichroic Cassegrain Subreflector”, IEEE Trans. on Antennas and Propagation, vol. AP-27, No. 4, pp. 466-473, July 1979). In these FSS applications, the geometrical properties of the array, such as its period and its geometrical shape, generate resonances in which the electromagnetic field is reflected or transmitted, and the surface concerned is then reflective or transparent. FSS are mainly used in applications employing multiband reflector antennas because these FSS use a single main reflector associated, as a function of frequency band, with a plurality of sources that are not placed at the same location but that, by means of different FSS type subreflectors, direct the electromagnetic field onto the main reflector whilst being transparent outside its operating band. There is therefore no phenomenon of masking if radiation in one frequency band intercepts a sub-reflector of another frequency band.
However, although they can take account of both types of polarization, these reflectors are not configurable in that they do not have exactly the same reflectivity for both polarizations in the same area.
To obtain a configurable FSS reflector, the paper by J. A. Bossard, D. H. Werner, T. S. Mayer, R. P. Drupp, “A Novel Design Technology for Reconfigurable Frequency Selective Surface using Genetic Algorithms”, IEEE Trans. on Antennas and Propagation, vol. AP-53, No. 4, pp 1390-1399, April 2005, proposes introducing switchable elements between each end of the crosses in order to produce an array of two sets of composite parallel lines intersecting at 90° and comprising discontinuous conductive strips separated by a component whose conductivity can be switched by application of a switching signal, such as a DC voltage, with switchable components consisting of PIN diodes. Accordingly, by imposing a given conduction state on the line segments between two consecutive intersection points of the array, it is possible to define runs of a plurality of vertical and horizontal segments having a given reflectivity. This results in a variation of the size of the basic pattern of the array, enabling the FSS resonant frequency to be adjusted in use, without it being necessary to change FSS. To modify the geometrical characteristics, it suffices to switch appropriately only some of the components.
However, the above-mentioned paper does not provide any information about how to apply the switching signal to the components in practice, except for applying a signal individually to each component, which would result in extremely complex connections, possibly even incompatible with the constraint of maximizing transmission by the reflector.
Moreover, since the operation of those known FSS applications, with and without configurability of the basic pattern, is based on the resonance or non-resonance of the array, they rely on the shape of the pattern and the period of the array to reflect or transmit electromagnetic waves in narrow frequency bands.